Remove spurious control-flow edge around switch block without a test-expression

Previously we thought it possible to get from top to bottom of a block like "switch { case f(): ... }", when in fact this is only possible if there are no case blocks to execute.

I also add tests for two possible corner cases of a switch without a test-expression: a completely empty switch (the 'true' is indeed the last node) and switch with an empty default block (a single 'skip' is generated for the default block and the 'true' is not the last node)

Author: smowton

ql/src/semmle/go/controlflow/ControlFlowGraphImpl.qll

@@ -1772,9 +1772,6 @@ module CFG {
       (
         lastNode(this.(ExpressionSwitchStmt).getExpr(), last, cmpl)
         or
-        last = MkImplicitTrue(this) and
-        cmpl = Bool(true)
-        or
         lastNode(this.(TypeSwitchStmt).getTest(), last, cmpl)
       ) and
       (
@@ -1783,6 +1780,10 @@ module CFG {
         not exists(this.getDefault())
       )
       or
+      last = MkImplicitTrue(this) and
+      cmpl = Bool(true) and
+      this.getNumCase() = 0
+      or
       exists(CaseClause cc, int i, Completion inner |
         cc = this.getCase(i) and lastNode(cc, last, inner)
       |

ql/test/library-tests/semmle/go/controlflow/ControlFlowGraph/ControlFlowNode_getASuccessor.expected

@@ -1206,7 +1206,6 @@
 | tst.go:3:12:3:12 | argument corresponding to x | tst.go:3:12:3:12 | initialization of x |
 | tst.go:3:12:3:12 | initialization of x | tst.go:4:2:4:2 | true |
 | tst.go:4:2:4:2 | true | tst.go:5:7:5:7 | x |
-| tst.go:4:2:4:2 | true | tst.go:12:1:12:1 | exit |
 | tst.go:5:2:5:13 | skip | tst.go:12:1:12:1 | exit |
 | tst.go:5:7:5:7 | x | tst.go:5:11:5:12 | 23 |
 | tst.go:5:7:5:12 | ...<... | tst.go:5:7:5:12 | case ...<... |
@@ -1232,12 +1231,11 @@
 | tst.go:9:12:9:12 | ...<... is false | tst.go:12:1:12:1 | exit |
 | tst.go:9:12:9:12 | ...<... is true | tst.go:9:2:9:13 | skip |
 | tst.go:14:1:14:1 | entry | tst.go:14:13:14:17 | argument corresponding to value |
-| tst.go:14:1:21:1 | function declaration | tst.go:0:0:0:0 | exit |
+| tst.go:14:1:21:1 | function declaration | tst.go:23:6:23:11 | skip |
 | tst.go:14:6:14:11 | skip | tst.go:14:1:21:1 | function declaration |
 | tst.go:14:13:14:17 | argument corresponding to value | tst.go:14:13:14:17 | initialization of value |
 | tst.go:14:13:14:17 | initialization of value | tst.go:15:2:15:2 | true |
 | tst.go:15:2:15:2 | true | tst.go:16:7:16:11 | value |
-| tst.go:15:2:15:2 | true | tst.go:21:1:21:1 | exit |
 | tst.go:16:2:16:34 | skip | tst.go:21:1:21:1 | exit |
 | tst.go:16:7:16:11 | value | tst.go:16:15:16:33 | ...*... |
 | tst.go:16:7:16:33 | ...<... | tst.go:16:7:16:33 | case ...<... |
@@ -1254,3 +1252,12 @@
 | tst.go:18:15:18:38 | ...*... | tst.go:18:7:18:38 | ...<... |
 | tst.go:18:38:18:38 | ...<... is false | tst.go:21:1:21:1 | exit |
 | tst.go:18:38:18:38 | ...<... is true | tst.go:18:2:18:39 | skip |
+| tst.go:23:1:23:1 | entry | tst.go:24:2:24:2 | true |
+| tst.go:23:1:25:1 | function declaration | tst.go:27:6:27:11 | skip |
+| tst.go:23:6:23:11 | skip | tst.go:23:1:25:1 | function declaration |
+| tst.go:24:2:24:2 | true | tst.go:25:1:25:1 | exit |
+| tst.go:27:1:27:1 | entry | tst.go:28:2:28:2 | true |
+| tst.go:27:1:31:1 | function declaration | tst.go:0:0:0:0 | exit |
+| tst.go:27:6:27:11 | skip | tst.go:27:1:31:1 | function declaration |
+| tst.go:28:2:28:2 | true | tst.go:29:2:29:9 | skip |
+| tst.go:29:2:29:9 | skip | tst.go:31:1:31:1 | exit |

ql/test/library-tests/semmle/go/controlflow/ControlFlowGraph/tst.go

@@ -19,3 +19,13 @@ func check2(value int64) {
 
 	}
 }
+
+func check3() {
+	switch { }
+}
+
+func check4() {
+	switch {
+	default:
+	}
+}